The present invention relates to a system for controlling the intensity of light which an exposing lamp emits to illuminate an original document in an image reading apparatus or a copying machine.
In an image reading apparatus or a copying machine, the light intensity of the illuminating lamp affects the image processing quality and image signal reading, and, in a copying operation, the image signal level, contrast and recording density. Such apparatus is thus designed to allow adjustment of the light intensity so as to match it with the properties of images of different original documents. A halogen lamp usable as an exposing lamp has a light intensity which varies in proportion to the 3.8th power of the voltage of a power source. Therefore, the source voltage is another factor which has influence on the image processing quality. To solve this problem, a prior art control system employs a thyristor such as a TRIAC connected in series with a halogen lamp, the lamp having a rating on the order of 80 V. This system makes the power supply voltage (effective value) about 80 V by controlling the power supply phase angle of the thyristor and automatically controls the phase angle of the power supply by advancing the phase when the source voltage drops and retarding it when the source voltage rises. Usually, an apparatus for such control is provided with a volume control (variable resistor) and a discrete circuit. The volume control is furnished with graduations indicating recording densities or light intensities for example. The discrete circuit employs a voltage corresponding to the phase angle selected through the volume control as its target value and shifts the phase angle in the advancing direction when the source voltage drops, shifts it in the retarding direction when the source voltage rises and triggers the TRIAC when the phase of the source voltage coincides with the phase angle. It is usual to use an analog discrete circuit partly because the indication output of the volume control is an analog signal and partly because the source voltage is analog.
This type of lamp control system involves various disadvantages. First, despite the fact that the volume control is manipulatable steplessly throughout its predetermined range, such fine adjustment is reflected only by insignificant differences in the actual results of image processing. This makes untrained operators feel it rather troublesome to select a specific position of the volume control. Second, after one operator used a copying machine or a facsimile transceiver with an image scanning device with a certain setting of the volume control, the next operator may happen to use it without altering the volume control setting. Where the volume control setting by the first operator is excessive for a document which the second operator intends to process, the resultant image will be poor in quality and the second operator will thus be required to copy or transmit the same document again after re-setting the volume control. A possible expendient for solving this problem is a resetting mechanism which, using a ball latch hving a spring biased steel ball for example, restrains the rotation of the volume control with a certain magnitude of force (the magnitude being such that it permits rotation for adjustment but overcomes the spring force which tends to return the volume control) while the power source is being turned on. After the power source has been turned off or just after the turning on of the power source, the resetting mechanism returns the volume control to its standard position by the action of the spring. However, this mechanism is very intricate in construction and needs a large number of parts and a large space. Generally, the standard position mentioned is an intermediate point in the adjustable range of the volume control or its neighborhood and, hence, the resetting mechanism requires a complex arrangement capable of returning the volume control in the opposite direction to the standard position. This objectionable in view of the fact that, in recent years, an increase in the number of mechanical elements costs more and involves a larger loss in space than an increase in the number of electric and electronic elements.